Subsequent to my previous question regarding watchdog timers, I am interested in deciding the RESULT of such a watchdog:
Assume I add a generic external watchdog timer to my circuit. If not toggled by my microcontroller within the watchdog timeout period, the watchdog will assert a Reset pin LOW for a few-millisecond period, then deasserts the signal.
Now, with this watchdog output signal, I have two options:
- Option A: I could apply it to the Reset pin of microcontroller
- Option B: I could connect it to the Enable-pin of my Voltage regulator which, in this case, thus resets the power for my entire circuit
My question: Are there any possible concerns with Option B? I have never seen a watchdog timer / reset-IC connected to a regulator in this fashion, whether in a watchdog timer datasheet or elsewhere. However, I find it desirable because that way, I would be able to "restart" all parts/ICs on my board, instead of just the microcontroller.
Best Answer
Using an external watchdog circuit to cycle the power to all your on-board circuity is a rather good idea. It helps to cover cases where some stray external event has caused an upset on some silicon chip circuit. Sometimes these events, whether they be high voltage spikes, reverse bias on signal inputs or even alpha particles can cause localized latchup on-board the chip and the only realistic way to clear such fault is to cycle the power.
Using the scheme to connect the WDT to the enable of the voltage regulator does require some due diligence on your part.
a) Make sure the WDT circuit is not powered by the same voltage rails as the disabled regulator.
b) Make sure that the length of time that the regulator is disabled is long enough. Bypass and filter capacitors on the output of the regulator need to discharge to the greatest degree possible in order to qualify as a power cycle to the silicon. In many instances a millisecond or so of disable time will only allow a partial discharge of the output rails.